Chancellor Philip Hammond had promised in August that the government would make up EU funding shortfalls up to 2020, which could cost the Treasury £6bn ($7.8bn, €6.9bn) a year.

But Thornberry, backed by Shadow Chancellor John McDonnell, will reveal that Labour will go further, with a promise of post-2020 funding.

"For the period 2014 to 2020, the UK was allocated €10.8bn in structural funding for our most deprived regions and communities," Thornberry will say.

"The Tories have given an undertaking hedged in conditions that funding up to 2020 will be protected.

"For the period after, they have said nothing. That is not good enough. Without long-term certainty over funding, our most deprived regions and communities cannot plan ahead. They cannot attract other investment. They cannot make progress.

"So thanks to John McDonnell, Labour's shadow chancellor, we can guarantee that a future Labour government will make up any shortfall in structural funding into the 2020s and beyond. And the same will go for the funding of peace and reconciliation projects in Northern Ireland."

Wales, which voted to leave the EU, is set to benefit most from the proposal. The region has been allocated €2.4bn between 2014 and 2020, while the South West has been allocated €1.4bn over the same period.

But the Tories have warned that Corbyn's re-election as Labour leader on Saturday has left the party "divided, distracted and incompetent".